In the manufacture of various sheet metal components, such as in the production of vehicles, a fabricated assembly is often subjected to heat treatment at temperatures in excess of 100° C. subsequent to coating such as by dipping or spraying. In certain portions of these assemblies, such as automobile body assemblies, door frames, and the like, the assembly is formed to provide a hem flange along one or more edges in order to strengthen the frame structure. A hem flange is formed by bending an edge portion of the panel inwardly about 180° so that the bent-in portion is folded back along the adjacent panel margin. This forms an open slot. Usually, another sheet metal piece, often called a stiffener, has a matching portion placed in the slot. This may then be spot-welded.
The frame portion with the hem flange so formed is subsequently passed through one or more dip tanks for cleaning purposes. The liquids used in the tank may include phosphate solutions, permeates and rinse water for example. This treatment may leave liquid contaminants on the panel surfaces, as well as in partially exposed voids such as those contained in hem flanges.
Most of these liquid contaminants are removed at a drying station along the assembly line. However, where the liquid has accumulated in voids, such as in a hem flange, the drying process is often not complete. Thus, when the frame assembly is passed through a heat-treating station with temperatures in excess of 100° C., the liquid may boil off and leave defects on one or more surfaces that must be removed by a manual procedure, such as sanding. Obviously, this is unsatisfactory and should be avoided.
Boiling off of the liquid contaminants occurs because the particular volume of liquid is heated to a temperature at or above its boiling point at the prevailing atmospheric pressure. When this occurs, the saturated vapor pressure within the volume of liquid equals the surrounding atmospheric pressure. Thus, the temperature of the liquid itself causes bubbles of vapor to form within the volume of liquid and then escape to the atmosphere. Boiling continues until the entire volume of liquid is converted to vapor. This results in defects on frame assembly surfaces as described above.
On the other hand, where the vapor pressure inside the volume of liquid is less than the pressure of the surrounding atmosphere (i.e. below the boiling point) then bubbles will not form and only ordinary evaporation will occur. Since ordinary evaporation is the preferred way to remove liquid contaminants, it would be desirable to keep the temperature of the liquid contaminant below the boiling point, but at a relatively high temperature to encourage evaporation.
The rate of evaporation is a function of temperature and pressure. The optimum rate of evaporation (without boiling) occurs when the temperature is relatively high and the pressure is relatively low. In the heat-treating process, the temperature must be at or above the boiling point and the atmospheric pressure is normally not adjustable.
Efforts have been made to remove these contaminants before they boil off in the heat-treating process, but the methods tried have significant disadvantages. Among the techniques tried are: (1) seal welding along all hem flanges, (2) applying an expandable weld sealer, (3) blowing compressed air into the flanges, and (4) shaking the panel during its travel along a conveyor. The first two techniques are cost-prohibitive and the last two have not been successful.
Accordingly, a need exists for a reliable and cost effective method of removing liquid accumulations from voids in hem flanges, before they boil off in the heat-treating process.